The invention relates generally to optical sensors. More particularly, the invention relates to interferometric sensors for determining optical path length.
As fiber optics become more prevalent, various types of optical sensors have become increasingly common. Indeed, various types of sensors can be used to detect fiber lengths, locations of breaks, cracks or inconsistencies in optical fibers, temperature, pressure, fiber expansion, attributes of chemical species, etc.
Optical fibers may be subjected to various external effects that produce geometric (e.g., size, shape) and/or optic (e.g., refractive index, mode conversion) changes to the fiber depending upon the nature and the magnitude of the perturbation. While these effects are often considered to be parasitic (i.e. noise-causing) in communications applications, the response of the fiber to external influence may be increased in sensing applications so that the resulting change in optical characteristics can be used as a measure of the external effect. Therefore, optical fibers may act as transducers that convert effects such as temperature, stress, strain, rotation or electric and magnetic currents into corresponding changes in optical effects.
Since amplitude or intensity, phase, frequency, and polarization typically characterize light, any one or more of these parameters may undergo a change due to external effects. The usefulness of the fiber optic sensor therefore depends upon the magnitude of this change and upon the ability to measure and quantify the change reliably and accurately.
Different types of sensors based upon fiber optic technologies are known. Among such sensor technologies are interferometers, which typically detect various phenomena by sensing phase changes or interference patterns between multiple optical signals passing through the sensor. In fact, interferometers can be used to determine distance, slope, rotation, and the like. Specifically, since about 1980, interferometric fiber optic gyroscopes (IFOGs) have been widely used to detect rotation, because such sensors have proven to be particularly useful for generating inertial navigation data that can be used to guide aircraft, automobiles, downhole drilling apparatus, and robots. Various embodiments of IFOGs are generally described in U.S. Pat. Nos. 6,211,963 and 6,175,410, which are incorporated herein by reference. In addition, techniques for sensing proper frequency used in conjunction with IFOGs are generally described in U.S. Pat. No. 5,734,469, which is incorporated herein by reference.
In practice however, interferometers are often complex and difficult to design and manufacture, and interferometers are typically not suitable for low-cost applications such as fiber optic length sensors. Thus, the present invention solves this problem by presenting a relatively simple and low-cost interferometric sensor that is accurate, has a high resolution, is useful for a variety of applications.
In accordance with one aspect of the invention, a sensor for determining a length of an optical path, comprises a light source, a modulator configured to direct light along said optical path, and an electronic system. The modulator is optically coupled to the light source, and the modulator is configured to modulate at least a portion of the light as a function of a modulation signal. The detector is optically coupled to the modulator and is configured to produce a detector output based upon a sensed intensity of the light at the end of the optical path. The electronic system is configured to receive the detector output, whereby the optical path length is determined by the detector output.
In accordance with another aspect of the invention, a method of determining a length of an optical path comprises the following: generating a light along the optical path; splitting the light into a first beam and a second beam; modulating at least one of the first beam and the second beam in response to a modulation signal to induce a difference between the first beam and the second beam; re-combining the first beam and the second beam to generate a recombined signal; detecting an output intensity of the recombined signal at a detector; adjusting the modulation signal as a function of the output intensity; and computing the length of the optical path as a function of the modulation signal.
In accordance with a further aspect of the invention, a system for determining the length of an optical fiber under test comprises a low coherence white light source configured to send light along an optical path defined by an optical fiber, a phase modulator optically coupled to the light source for modulating at least a portion of the light in a first path relative to a second path, a detector optically coupled to the optical path for producing a detector output based upon a length of the optical path, and a processor for receiving the detector output and for producing an output based upon the length of the optical path, wherein the light source has a coherence length shorter than a difference in path length between the first and the second paths.